To render a transmission device, the shaft of a fan for example, being supported and positioned, a bearing is generally used for the insertion and mounting of the shaft such that the shaft may rotate smoothly. Conventional bearing is in general divided into ball bearing and oil-impregnated bearing, which are described as follows:
(1) Ball bearing assumes a multiple-point contact with shaft through a multiple of balls rolling to form a rolling friction. The advantage of such a contact is low friction coefficient; whereas its disadvantage lies in the fact that when one of the balls in a ball bearing fails, the shaft cannot rotate or rotates un-smoothly so as to produce noises or damage the shaft. In addition, a ball bearing with its complex structure may result in a shortened service life as a result of changing environmental conditions, such as excessive temperature or external stress. Further, a ball bearing consists of a lot of fine components, leading to a higher cost and thus affecting the production cost of the whole transmission device.
(2) The manufacture of an oil-impregnated bearing first goes through a pressing process. Processed metal particles are delivered into the cavity of a precision mold and the upper and lower molds are then compressed to form a specific shape of green body, complement to that of the mold cavity. The green body is then sintered, for example, in a flowing protective atmosphere at an appropriate temperature relative to the melting temperature of its main constituent, two thirds of the copper's melting point for example. After an appropriate duration of time, the particles of the green body are sintered to form a sintered body or semi-finished product with a multiple of dense voids. To impregnate oil into the sintered body, it is placed in a container having liquid lubricant and vacuumed to enable the oil entering into the multiple dense voids within the sintered body so as to form an oil-impregnated bearing.
Since oil-impregnated bearings are inexpensive and thus are used extensively. However, the shortcoming of an oil-impregnated bearing lies in the fact that it assumes a planar contact with a shaft, leading to a sliding friction, which has a higher friction coefficient. Also, the volume of the dense voids is insignificant and thus the amount of oil stored is limited; its heat conductivity is unsatisfactory. Further, high temperature resulted from friction sometimes renders the lubricating oil degraded, causing carbonization and carbon clogging. Dusts and wear debris resulted accumulated so as to reduce the lubrication between bearing and shaft, and heat expansion finally cause the bearing and shaft being stuck.
Thus the application of the conventional bearings has been restricted significantly, and further improvements are needed to achieve low friction coefficient, high wear resistance, enhanced oil storing capacity, and extend the service life of bearings.